It is known to test one or more capacitive measuring microphones associated with preamplifiers and connecting wires, where each microphone is connected to the inlet of a preamplifier having a relatively high input resistance. The microphones may be placed far away from a central control unit performing the testing optimally automatically. A test signal is transmitted through an individual test conduit from the central control unit to each microphone. By this known method, the test conduit is connected to the chassis terminal of the microphone. The test signal is transmitted through the capacity of the microphone. Modern preamplifiers ensure that the response to the test signal is almost independent of the capacity of the microphone, and accordingly it is not possible to detect changes therein although said capacity has become extremely high or optionally shortcircuits.
Elder preamplifiers with a relatively high input capacity provide a slight sensitivity to changes in the capacity of the transducer. This sensitivity is, however, very small, typically of a magnitude smaller than the one achievable by the method according to the invention. In addition, external electromagnetic fields presents a technical problem, especially in connection with large measuring systems. The measuring system associated with the known central control unit is sensitive to external fields due to the separate chassis terminals of the microphone and the preamplifier.
U.S. Pat. No. 4,648,078 discloses a system for testing a plurality of acoustic transducers, a test signal being transmitted through the capacity between the screen and the inner conductor of a screened cable to the joint between the individual transducer and an amplifier. This system is, however, only able to indicate whether the amplifier and the cable to a central control unit is in order. The system is unable to indicate whether the capacity of the transducer has changed.